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You may remember learning that there are three primary colors. That number is very significant

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It turns out that to see the different colors we see we have to have different types of receptors to catch light and

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that these receptors must be most sensitive to different wavelengths of light

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Those who have normal color vision have three different classes of receptors in our cones one specialized for long (red) wavelengths,

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one specialized for medium (green) wavelengths, and one specialized for short (blue) wavelengths

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This theory is called the trichromatic three color theory.  The theory is also called the Young/Helmholtz theory after the scholars who first propsed the theory; Thomas Young and Herman Von Helmholtz

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The trichromatic theory explains how we see different colors depending on the strength of the responses from different cones

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To see blue, you need to have mostly the short wavelength cone respond - it is similar for green and red

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Other colors that are not primaries require a balance of responses across some of the cones

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To see yellow you need to have a balance of response from both the middle and long wavelength cones

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When all three types of cones respond at a high rate we see white, we see black when none of the cones types are responding much at all

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When you mix colored lights as on a computer screen you add different wavelengths together and more wavelengths enter the eye

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Notice that white requires a balance of all three primary additive colors

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When you mix colored paints or other pigments you subtract wavelengths from the light reflected into the eye because they are absorbed by the object

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Fewer wavelengths enter the eye then were present in the original light

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if you mix enough different pigments all wavelengths will be absorbed and you will experience black